Risk Factors and Sonographic Diagnosis of Vasa Previa
Vasa previa is the presence of fetal blood vessels at or near the internal cervical os. These vessels are exposed to spontaneous rupture or compression and rupture at the time of amniotomy or labor. The total fetal blood volume is relatively small, and blood loss from vessel rupture can result in exsanguination and death.
Risk factors and sonographic diagnosis of vasa previa.
Above. Color Doppler image of vasa previa. The cervix is demonstrated and the region of the ICO (internal cervical os). The vessels are seen traversing this area and are confirmed to be fetal in origin by Doppler velocimetry.
Etiology of Vasa Previa:
A. Velamentous insertions near the internal cervical os: The cord inserts into membranes and exposed vessels cross the internal cervical os (Occurrence: 25% to 62%).
B. Exposed vessels cross the internal cervical os between a separate extra lobe of the placenta (succenturiate or bilobed) and the main placental mass (occurrence: 33% to 75%).
C. Second-trimester placenta previa: Involution of a portion of placenta previa leaves vessels exposed over the internal cervical os. (Occurrence up to 69%).
• 1 in 2000 to 1 births
• Recent retrospective series (1 in 1310 births)
Risk Factors / Odds Ratio (OR)
• IVF Pregnancy / 7.75 (OR)
• Bilobed or succenturiate placenta / 22.11 (OR)
• Second trimester placenta previa / 22.86 (OR)
• Velamentous cord insertion
• Cord insertion into lower 1/3 of the uterus (occurs 11% of the time but its occurrence is associated with placental malformation and accessory placenta)
• Late pregnancy bleeding
Above. A summary of risk factors is presented.
Assess carefully the following group of patients, those with:
• Grayscale echogenic parallel or circular lines (umbilical cord) near the internal cervical os.
• Velamentous cord insertion particularly if low cord insertion site.
• Extra lobe of the placenta, especially if in the lower 1/3 of the uterus.
• History of second-trimester placenta previa.
• Low cord insertion site.
If any of the above findings are present, perform color Doppler transvaginal ultrasound of the internal cervical os region
Ultrasound Accuracy and Vasa Previa
A recent systematic review of the accuracy of ultrasound in the diagnosis of vasa previa included 583 articles of which two were prospective and six were retrospective cohort studies. Whether transvaginal ultrasound (TVS) is used for primary evaluation or for evaluation after vasa previa is suspected on the transabdominal scan, TVS with color Doppler is the superior method for evaluation. Transvaginal color Doppler in the second trimester resulted in 100% sensitivity and 99.0% to 99.8% sensitivity.
3-D in Diagnosis of Vasa Previa
3-D ultrasound may complement the diagnosis:
Look for multiplanar and surface rendered views of fetal vessels over the internal cervical os.
Assess 3-D axial view of endocervical canal and visualize fetal vessels in relationship to internal cervical os.
In undetected cases of vasa previa, fetal mortality has been reported as high as 30% to 100% due to rupture of fetal vessels and fetal blood loss.
Mortality depends upon the antenatal diagnosis. Recent data, cited above, suggests survival is as high as 97% if the diagnosis is made antenatally while the survival is lower if the diagnosis is not made.
Prevention of fetal death:
• Prenatal diagnosis by ultrasound is essential. C-section delivery should be performed before the onset of fetal membrane rupture with the potential for fetal umbilical vessel rupture. Recommended gestational age for preventive C-section is not uniform, but some authors suggest 35 weeks gestation in the absence of ruptured membranes or labor.
Lee, et al studied 18 cases of vasa previa screened by ultrasound with overall excellent outcomes. In that report, there were 2 deaths, one fetal and one neonatal.
• High index of clinical suspicion prior to amniotomy is needed since not all cases are diagnosed by ultrasound before birth. This is particularly true in monochorionic diamniotic twins and some IVF-conceived twins or IVF-conceived higher-order multiples.
• Type II Vasa Previa (from bilobed or succenturiate lobe placenta ) has been successfully treated with an in-utero laser.
• Newer methods of detecting fetal hemoglobin have been described, which could aid early diagnosis of clinical vasa previa.
Vasa previa occurs when sub-membranous fetal vessels cross the internal cervical os.
Summary and Other Considerations
1. Velamentous cord insertion occurs with vessels traversing membranes, which cross over or near the internal os.
2. Bi-lobed placenta is present and fetal vessels traverse membranes from one lobe to the other lobe and cross over the internal os or near the internal os.
3. History of mid-trimester placenta previa occurs, the marginal placental mass is absorbed and there is membranous fetal vessel exposure over or near the internal os.
1. On the transabdominal grayscale, echogenic parallel or circular lines may appear near the internal cervical os, which likely represents the umbilical vessels.
2. Carefully observe cord insertion site, particularly if there is placental implantation in the lower one-third of the uterus.
3. Assess for the possibility of an accessory lobe or a succenturiate lobe.
4. Distinguish between marginal cord insertion and velamentous cord insertion.
5. Observe the cervical vascular arrangement in patients with the placental margin near the internal cervical os.
Confirmation of the diagnosis:
1. Perform transvaginal ultrasound.
2. Use color Doppler to identify fetal vessels.
3. Place pulsed Doppler gate within fetal vessels and confirm fetal origin on Doppler velocimetry.
Pitfalls in the diagnosis:
1. The umbilical cord may be presented in the lower uterine segment and near the cervix, but no sub-membranous vessels are traversing the internal cervical os.
2. Placental implantation near the internal cervical os may exhibit maternal veins.
3. Maternal cervical vessels near the internal cervical os can be observed with color Doppler and can be mistaken for fetal vessels.
In all of the cases cited above, pulsed Doppler velocimetry will distinguish maternal vasculature from fetal vasculature.
Above. Patient 1. 21 2/7ths weeks. 2-D grayscale. Longitudinal view. PL (placenta) demonstrating velamentous CI (cord insertion). Note AL (accessory lobe), CR (cervical region), and maternal BL (bladder).
Above. Patient 1 (continued). Color Doppler image demonstrating velamentous CI (cord insertion).
Above. Patient 1 (continued). Color Doppler image, again demonstrating VCI (velamentous cord insertion). Again, note AL (accessory lobe) and CR (cervical region).
Above. Patient 1 (continued). Transvaginal ultrasound. 2-D grayscale. Sagittal view demonstrating ICO (internal cervical os). A sub- membranous fetal vessel is identified near the ICO.
Above. Patient 1 (continued). Transvaginal ultrasound. Sagittal view. Color Doppler image confirming the fetal vessel seen in the grayscale view.
Above. Patient 1 (continued). 3rd trimester. Transvaginal ultrasound color Doppler image suggests a fetal vessel. Pulsed Doppler velocity waveform confirms the presence of a fetal arterial vessel and substantiates the diagnosis of vasa previa, which was observed following delivery by C-section.
Vasa Previa, Patient 2
Above. Patient 2. 25 weeks. 2-D grayscale. Longitudinal view. Note 2 separate placental masses.
Above. Patient 2 (continued). Color Doppler image demonstrating VCI (velamentous cord insertion). Note the AL (accessory lobe) and the apparent cord insertion site between the anterior placenta and the accessory lobe.
Above. Patient 2 (continued). Color Doppler image demonstrating VCI (velamentous cord insertion). Note the cord insertion site is near the cervical region.
Above. Patient 2 (continued). Color Doppler image with the transverse view of the cervix demonstrating sub-membranous insertion of the fetal vessel near the cervix.
Above. Patient 2 (continued). Transvaginal ultrasound view, color Doppler image confirming the fetal vessels inserting near and crossing over the ICO (internal cervical os). Pulsed Doppler confirmed the fetal origin of the vessels.
Above. Patient 3. 29 2/7 weeks. 2-D grayscale. Transverse view. Note 2 separate placental masses with apparent cord insertion into the margin of the anterior placenta.
Above. Patient 3 (continued). 2-D grayscale. Longitudinal view of lower uterine segment and cervix. Note that the leading edge of the main placental mass is 1.3 cm from the ICO (internal cervical os).
Above. Patient 3 (continued). Transvaginal ultrasound, 2-D grayscale image suggesting fetal vessels are crossing the ICO (internal cervical os).
Above. Patient 3 (continued). Transvaginal ultrasound view, color Doppler image. Fetal vessels connect the accessory placental lobe to the placenta while crossing at or near the internal cervical os.
Above. Patient 3 (continued). Transvaginal ultrasound view, color Doppler image. Pulsed Doppler ultrasound confirms that the vessels in question are fetal vessels crossing at or near the internal cervical os.
Vasa Previa, Patient 4
Above. Patient 4. Twin gestation with the posterior placenta and suspected VCI (velamentous cord insertion). Transvaginal ultrasound, 2-D grayscale demonstrates the margin of the placenta to be near the cervix and the velamentous cord inserting into the placental margin near the cervix.
Above. Patient 4 (continued). Transvaginal ultrasound view, color Doppler image demonstrates the VCI (velamentous cord insertion) into the placental margin and one of the UA (umbilical arteries). The sub-membranous fetal vessels are crossing the cervix near the internal cervical os.
Above. Patient 4 (continued). Transvaginal ultrasound view, color Doppler image. The pulsed Doppler ultrasound waveform confirms the presence of a fetal arterial vessel at or near the internal cervical os.
Vasa Previa versus Umbilical Cord, Patient 5
Above. Patient 5. 2-D grayscale. Transverse view. Note 2 separate placental masses with an anterior placenta and a possible accessory lobe. There are multiple free loops of UC (umbilical cord).
Above. Patient 5 (continued). Transvaginal ultrasound, 2-D grayscale demonstrates the margin of the placenta to be near the cervix. There is the possibility of an FV (fetal vessel) on grayscale. Alternatively, the fetal vessel could represent a retro-placental vascular space of maternal origin.
Above. Patient 5 (continued). Transvaginal ultrasound view, color Doppler image demonstrates an FV (fetal vessel) near the ICO (internal cervical os) and a loop of UC (umbilical cord). The goal is to demonstrate the difference between a free loop of an umbilical cord and a sub-membranous fetal vessel over the ICO.
Above. Patient 5 (continued). Transvaginal ultrasound view, color Doppler image. The pulsed Doppler gate should be placed when there is a simultane
ous view of the internal cervical os on transvaginal ultrasound. In this image, the placement is not correct.
Above. Patient 5 (continued). Transvaginal ultrasound view, color Doppler image. Again, the pulsed Doppler gate should be placed when there is a simultaneous view of the internal cervical os on transvaginal ultrasound. While the waveform is fetal in origin, the possibility of an umbilical cord vessel versus a sub-membranous vessel over the os cannot be delineated in this view.
Above. Grayscale image, longitudinal view demonstrate the UC (umbilical cord) and the ICO (internal cervical os). Transvaginal ultrasound with color Doppler and pulsed Doppler is necessary to define vasa previa.
Above. 2-D grayscale demonstrating the ICO (internal cervical os) and the placenta near the ICO. Marginal placental vascular structures are common in this location, and pulsed Doppler may demonstrate maternal arterial or venous structures.
Above. Pulsed Doppler demonstrates a vessel near the cervix with a rate of 75 bpm consistent with maternal origin.
Risk Factors and Sonographic Finding Placenta Accreta
Risk Factors and Underlying Pathology
Terminology: placenta accreta is currently labeled placenta accreta spectrum (PAS) disorder.Morbidly adherent is another frequently used term.
Above. The pathology of placenta accreta is related to a defect in the decidua basalis caused by uterine scaring, and a variety of surgical procedures may be responsible for the defect.
Above. The recent increase in cesarean birth and repeat cesarean births increase the overall incidence of anterior uterine scarring, and these trends translate into a significant effect on the incidence of placenta accreta.
Incidence and Risk Factors
Above. The incidence of placenta accreta is recorded according to recent studies and varies from almost 0.5% to almost 1%.
The incidence has progressively increased as the number of C-sections has increased, and placenta accreta is one of the greatest challenges in contemporary obstetrics.
Above. Most patients with placenta accreta will demonstrate one or more risk factors and, as expected, the highest risk factors are placenta previa and C-sections.
Other risk factors are defined and include: multiparity, previous curettage, history of myomectomy, endometrial ablation, hysteroscopy lysis of adhesions, and termination of pregnancy (unknown effect
Diagnostic and Sonographic Features
Recent papers have reviewed and clarified diagnostic and sonographic features of placenta accreta.
The overall sensitivity of sonography for placenta accreta diagnosis is 86.4% versus 84% for MRI.
The following is noted during the first trimester:
1. The presence of a lower uterine segment gestational sac.
2. Presence of irregular vascular spaces within the placenta.
3. Implantation of gestational sac within a lower uterine scar.
During the second and third trimester, the following are possible findings:
1. Multiple vascular lacunae within the placenta.
2. Loss of retroplacental hypoechoic zone. (Not confirmed by all studies.)
3. Abnormalities of the interface between the bladder and uterine serosa.
4. Villi extension into myometrium, serosa, or bladder.
5. Doppler demonstration of turbulent flow through lacunae.
When multiple observers are blinded from sonographic patients with and without placenta accreta, the diagnostic accuracy ranges from 55.9% to 76.4% with sensitivities from 53.4% to 74.4%, and overall, the inter-observer agreement is moderate.
Early Diagnosis of Placenta Accreta
At 10 weeks or less ultrasound demonstrates:
• Gestational sac in the lower uterine segment.
• Implantation of the sac over the uterine scar.
At 16 weeks or greater ultrasound demonstrates:
• Irregularly shaped vascular sinuses with color Doppler.
• Irregular or small bulge into the adjacent bladder.
• Placental sinuses that cross the bladder wall.
Transvaginal ultrasound and color Doppler imaging improve the accuracy of placenta accreta prediction in patients at risk.
The highest positive predictive value for placenta accreta is irregular vascular spaces within the placenta (placenta lacunae).
In patients with previous C-section, ultrasound findings for placenta accreta include:
1. Grayscale 2-D: multiple venous lakes (lacunae).
2. The presence or absence of lacunae is important in assessing the risk for maternal hemorrhage.
3. Color Doppler: vascularity with abnormal vessels linking placenta to the bladder and placenta to the myometrium.
4. Finally, less than 1 millimeter for myometrium thickness is associated with placenta accreta
Above. Transvaginal ultrasound image in a patient with previous C-section and placenta. Note small lacunae and the placental location over the internal cervical os (complete placenta previa).
Above. Transvaginal ultrasound demonstrates placental lacunae in a patient with placenta accreta.
Above. Note precise color Doppler imaging demonstrates increased vascularity. There is increased vascularity and penetration of placental vessels to the myometrium.
Above. Color Doppler imaging demonstrates increased vascularity and potential penetration of placental vessels to the myometrium.
Above. Note relatively smaller lacunae in a patient at risk for placenta accreta.
Above. Note relatively thin myometrium of 1.8 mm in a patient at risk for placenta accreta.
Above. Note the thin uterine wall and the large lacunae suggestive of placenta accreta.
Role of ultrasound in the assessment of placental abruption
Abruptio placenta is the separation of the normally implanted placenta from the myometrium or uterine wall.
• 1% to 2% of all pregnancies.
• 5.1% of preterm deliveries.
• Failure of fetal membranes to form normally at about 9 weeks.
• Severe growth restriction and abruption
• Chronic histologic changes in the placenta, umbilical cord, and membranes.
• Humoral immune response of the mother to the fetus suggested by IL-6 increase in patients with abruption.
• Thrombophilia (Factor II and Factor V Leiden may influence the course of placental abruption).
Important clinical associations are:
• Bleeding at 20 weeks relative risk 1.6.
• Premature rupture of membranes (PROM): RR = 3.58.
• Chronic hypertension: 15.6/1000 pregnancies versus 5.8/1000 non-hypertensive pregnancies.
• Severe growth restriction: 9 times as likely versus fetal weights > 90th percentile.
• Circumvallate and circummarginate placenta: odds ratio increased 13.1.
In the presence of maternal bleeding, placenta previa should be excluded. Transvaginal ultrasound can be useful early in gestation to define the extent of the hemorrhage. The amount, location, and size of the hemorrhage are important variables in defining outcome as well as maternal symptoms of pain.
If the bleeding is greater than 2.0 cm from the internal cervical os (ICO) at 19 weeks or less, look for findings associated with subchorionic hemorrhages, such as the presence of subchorionic fluid. At greater than 20 weeks gestation in the presence of maternal bleeding, retroplacental or intra-placental clots may not be seen. In both cases, clinical factors are important, such as duration, size, and location of the hemorrhage.
First trimester or early mid-trimester changes include:
• Extrachorionic and complex collections of fluid or blood.
• Crescent-shaped collections of fluid or blood.
• Absence of echoes in the subchorionic space.
Second trimester or third-trimester ultrasound findings include:
• Wide range of echogenicity depending upon timing.
• Acute bleeding that may be difficult to distinguish from placental tissue.
• Increased placental thickness.
• Rounded or separated placental edges.
Above left. First trimester subchorionic bleed with an anechoic collection of fluid and blood in the subchorionic space.
Above right. Mid-trimester, a pattern of mixed placental echogenicity with portions of subchorionic space are anechoic, while other portions demonstrate a mixed echogenic pattern. Heavy bleeding during the first trimester is often a risk factor for premature delivery and placental abruption.
Above. 11 6/7 weeks gestation. Note the anterior placenta and the amnion chorion separation from the chorionic plate. The anechoic area represents fluid and/or blood.
Above. First-trimester SCH (subchorionic bleed). The relative positions of the amnion and chorion are illustrated. There is an extensive anechoic collection of fluid and blood in the subchorionic space.
Above. At the margin of the placenta, the amnion chorion membrane is elevated. The mixed echoes suggest clot between the amnion and chorion while there is fluid or blood in the subchorionic space and some evidence of marginal acute bleeding.
Above. Extensive hematoma in a woman with chronic bleeding. This represents a mixed pattern with anechoic areas suggesting fluid or blood and the mixed echos suggesting clot formation.
Above. Again, extensive hematoma in a woman with chronic bleeding. This represents a mixed pattern with anechoic areas suggesting fluid or blood and the mixed echos suggesting clot formation. In addition, the small echogenic particles representing blood fragments within the amniotic fluid are consistent with a history of maternal bleeding.
Above. Mid-trimester scan in a patient with chronic bleeding. This likely represents a marginal placental abruption with the clots demonstrating differences in age. Note the hyper-echogenic area compared to the clot near the placental margin.
Above. Chronic abruptio placenta. Note placental texture compared to the clot with a mixed echogenic pattern near the ICO (internal cervical os).
Above. Mid-trimester scan in a patient with the low-lying posterior placenta and chronic bleeding. Note the pattern of mixed echogenicity and the extension to the internal cervical os (ICO) area is primarily clotted from the chronic abruptio placenta.